Utilization of marine sedimentary dissolved organic nitrogen by native anaerobic bacteria

Bioavailability and chemical composition of dissolved organic nitrogen (DON) in marine pore water were followed and related to bacterial activity in a 14-d anoxic decomposition experiment. For the experiment 0.2-mum filtered pore water from a coastal marine sediment was inoculated with native sediment bacteria. The initial composition of DON was characterized by a high contribution of dissolved free amino acids (DFAA), dissolved combined amino acids (DCAA), and urea, which together accounted for 82% of the DON pool. During the experiment, 54% of the DON was used by bacteria, indicating that DON was readily available to bacterial degradation. The consumption of DFAA and DCAA accounted for more than half of the DON consumed. The unidentified DON pool accounted for 33% of the net DON consumption, and the unidentified DON pool was fully depleted by the end of the experiment. Systematic changes in the amino acid composition occurred with time of incubation, as demonstrated by use of a principal component analysis based on the mole percent contribution of amino acids. The results indicated that amino acids of both the DFAA and the DCAA pools were useful indicators of the diagenetic state of DON. The present study presents the first direct evidence of short-term (within 14 d) changes in DFAA and DCAA of the DON pool. These changes were similar to decompositional changes in the molecular composition of amino acids observed in traditional studies of bulk sediment diagenesis.